The present invention concerns an electromagnetic rotary stepping motor comprising a stator formed of soft ferromagnetic material and provided with a single exciting winding, and a rotor comprising a magnet having at least one pair of poles, the stator and the rotor of the motor being magnetically coupled in a manner to give the rotor a preferred sense of rotation in response to a first type of single alternating polarity pulses applied to the winding.
Electromagnetic stepping motors of this type are already well-known. The rotor of such a motor is subject to two types of moments: a static holding moment due to the flux of the permanent magnet and which defines the stopped position of the rotor when the motor is not fed with current (this also known as the reluctance moment) and a dynamic moment of the motor due to the interaction between the flux of the magnet and the flux created by the winding when the latter is fed by a current (also known as the mutual moment magnet-winding). It is the dynamic moment which provides the motor with the energy necessary for its rotation. In order to use such a motor in a timepiece for example it is necessary to fashion it in a manner such that it turns always in one sense in response to single pulses of alternating polarity applied to the winding. Such is the case of the motor described in Swiss Pat. No. 604,250 in which the angular shift of the stator relative to the fixed angular member determined at all times a preferential sense for rotation of the motor. It follows from the foregoing that the signals generally utilized for operating this motor and which appear in the form of single pulses polarized once in one sense and once in the other sense are inapt to effect turning of the rotor in a sense opposite to that preferred.
The purpose of this invention is to drive the rotor of the motor in a sense opposite to that normally preferred, thereby to be able to turn at least one of the hands of the timepiece in a sense opposite to the normal sense. Such a movement may be desirable particularly if one should desire for example to rapidly reset the hour in a timepiece provided with an analogic type display or again, if in employing a recent invention which uses momentarily the minutes hand as a seconds hand, one should desire to return the display to the original time shown. In order to accomplish this one provides the motor winding with a composite signal which may be provided on demand through the operation of a control button.
Monophase electromagnetic bipolar motors are known in which a special signal is applied with the purpose of changing the sense of the rotation.
In the French Publication No. 2,373,817, there has been described a system in which a pulse is applied to the winding in phase opposition just before a normal pulse and detection means operate in order to detect the position of the rotor so as to pass from one pulse to the other at the right moment in order to reverse the rotation sense of the rotor. Such a system presents at least two difficulties: firstly it is reversible i.e. if following a shock the rotor should accidentally advance through a step the composite pulse following will cause the rotor to continue in the preferred sense. Further there is necessity for a sophisticated system of adjustment which controls the switching of the pulses.
In the U.S. Pat. No. 4,112,671 other means are described for reversing the rotation sense of the rotor, in particular a group of pulses 10,11 and 12 as shown in FIG. 5 of this patent is arranged to reverse the sense of rotation of the rotor. This control system is associated with a rotor having a small moment of inertia relative to the volume of the rotor. If the couple or torque developed by the motor is sufficient to drive the mehanism of the timepiece in a forward direction its value is reduced by three times when the reverse operation is engaged and one may conclude that this function is by no means reliable whereby steps may be lost. If such should not present a major difficulty at the time of resetting the time, it will be appreciated that the loss of a step in a watch where one utilizes the minutes hand as a seconds hand will bring about a loss of the real time when one wishes to return to displaying the minute.
In the German publications DEOS No. 2,808,534 and DEOS No. 2,628,583 there is likewise described an arrangement for reversing the rotation sense of a motor. In order to step the rotor in the sense opposite the preferred sense, the winding is fed with a first pulse the polarity of which effects rotation of the rotor through an angle less than that corresponding to a full step in the preferred sense. This is followed by a second pulse of opposite polarity which effects turning of the rotor through a step in the sense opposite to the preferred sense. Both of these publications are concerned with facilitating setting of a watch. The first-mentioned recognizes that the reverse operation provides less output then the forward and that steps may be lost. The second is silent on this subject and gives no guarantee of reliable operation in both senses of rotation.